fix(#3422): the bridge reads and writes users.yml directly

`swarm agent create` failed with "no user store at
swarm-authelia-bridge-users.json but users.yml already holds users" on
any hive that had users. The guard was correct; what was wrong is that
two files both claimed to be canonical for one physical file.

The bridge kept a private users.json and rendered users.yml from it,
while swarmctl kept its own pair against the same users.yml. A writer
whose own JSON was absent could not tell "nothing here yet" from
"someone else's users", so it refused to write at all.

users.yml becomes the store: read before write, through serde_norway
rather than a hand-rolled emitter. Unknown top-level and per-user keys
round-trip through `extra`, or whichever process writes second would
silently delete what the first added. Validation stays on the write path
-- a bare to_string(&store) serialises perfectly and drops the "no
control character ever reaches this file" guarantee silently.

The seed constant goes with the guard: nothing writes a seed now, an
absent file is an empty store, and left as a pub constant it read as if
the seed dance were still load-bearing.

swarm-authelia-bridge/Cargo.toml

@@ -14,6 +14,7 @@ axum.workspace = true
 reqwest.workspace = true
 serde.workspace = true
 serde_json.workspace = true
+serde_norway = "0.9.42"
 swarm-authelia-bridge-sock.workspace = true
 tokio.workspace = true
 tracing.workspace = true